Make use of set_default method for better code readability.

powerlevel9k.zsh-theme

@@ -211,11 +211,11 @@ prompt_aws() {
 prompt_battery() {
   icons[BATTERY_ICON]=$'\UE894'
   # set default values of not specified in shell
-  [[ -z $POWERLEVEL9K_BATTERY_CHARGING ]] && POWERLEVEL9K_BATTERY_CHARGING="yellow"
-  [[ -z $POWERLEVEL9K_BATTERY_CHARGED ]] && POWERLEVEL9K_BATTERY_CHARGED="green"
-  [[ -z $POWERLEVEL9K_BATTERY_DISCONNECTED ]] && POWERLEVEL9K_BATTERY_DISCONNECTED="$DEFAULT_COLOR_INVERTED"
-  [[ -z $POWERLEVEL9K_BATTERY_LOW_THRESHOLD ]] && POWERLEVEL9K_BATTERY_LOW_THRESHOLD=10
-  [[ -z $POWERLEVEL9K_BATTERY_LOW_COLOR ]] && POWERLEVEL9K_BATTERY_LOW_COLOR="red"
+  set_default POWERLEVEL9K_BATTERY_CHARGING       "yellow"
+  set_default POWERLEVEL9K_BATTERY_CHARGED        "green"
+  set_default POWERLEVEL9K_BATTERY_DISCONNECTED   "$DEFAULT_COLOR_INVERTED"
+  set_default POWERLEVEL9K_BATTERY_LOW_THRESHOLD  10
+  set_default POWERLEVEL9K_BATTERY_LOW_COLOR      "red"
   [[ -z $POWERLEVEL9K_BATTERY_FOREGROUND ]] && local fg_color="" || local fg_color="%F{$POWERLEVEL9K_BATTERY_FOREGROUND}"
   [[ -z $POWERLEVEL9K_BATTERY_FOREGROUND ]] && local conn="$DEFAULT_COLOR_INVERTED" || local conn="$POWERLEVEL9K_BATTERY_FOREGROUND"